"""Provides a mechanism for not competing with other processes interacting with an MSAL cache."""
import os
import sys
import errno
import time
import logging

import portalocker


logger = logging.getLogger(__name__)


class CrossPlatLock(object):
    """Offers a mechanism for waiting until another process is finished interacting with a shared
    resource. This is specifically written to interact with a class of the same name in the .NET
    extensions library.
    """
    def __init__(self, lockfile_path):
        self._lockpath = lockfile_path
        self._lock = portalocker.Lock(
            lockfile_path,
            mode='wb+',
            # In posix systems, we HAVE to use LOCK_EX(exclusive lock) bitwise ORed
            # with LOCK_NB(non-blocking) to avoid blocking on lock acquisition.
            # More information here:
            # https://docs.python.org/3/library/fcntl.html#fcntl.lockf
            flags=portalocker.LOCK_EX | portalocker.LOCK_NB,
            # Support for passing through arguments to the open syscall
            # was added in Portalocker v1.4.0 (2019-02-11).
            buffering=0,
        )

    def _try_to_create_lock_file(self):
        timeout = 5
        check_interval = 0.25
        current_time = getattr(time, "monotonic", time.time)
        timeout_end = current_time() + timeout
        pid = os.getpid()
        while timeout_end > current_time():
            try:
                with open(self._lockpath, 'x'):  # pylint: disable=unspecified-encoding
                    return True
            except ValueError:  # This needs to be the first clause, for Python 2 to hit it
                logger.warning("Python 2 does not support atomic creation of file")
                return False
            except FileExistsError:  # Only Python 3 will reach this clause
                logger.debug(
                    "Process %d found existing lock file, will retry after %f second",
                    pid, check_interval)
                time.sleep(check_interval)
        return False

    def __enter__(self):
        pid = os.getpid()
        if not self._try_to_create_lock_file():
            logger.warning("Process %d failed to create lock file", pid)
        file_handle = self._lock.__enter__()
        file_handle.write('{} {}'.format(pid, sys.argv[0]).encode('utf-8'))  # pylint: disable=consider-using-f-string
        return file_handle

    def __exit__(self, *args):
        self._lock.__exit__(*args)
        try:
            # Attempt to delete the lockfile. In either of the failure cases enumerated below, it is
            # likely that another process has raced this one and ended up clearing or locking the
            # file for itself.
            os.remove(self._lockpath)
        except OSError as ex:  # pylint: disable=invalid-name
            if ex.errno not in (errno.ENOENT, errno.EACCES):
                raise
